Process for tunnelized cyclostationary to achieve low-energy spectrum sensing

ABSTRACT

A system and method that uses tunnelizing for analyzing frequency spectrum. The method may include the steps of under sampling an input signal to take samples in one or more tunnels each with a tunnel bandwidth that is equal to or less than a total analysis bandwidth of the input signal; detecting one or more cyclostationary features of the input signal based on the samples; and determining one or more signal types of the one or more cyclostationary features based on the one or more cyclostationary features.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/761,958 filed Jul. 20, 2015, which claims priority to PCT ApplicationSerial No. US/2014/066180 filed Nov. 18, 2014, which claims the benefitof U.S. Provisional Application Ser. No. 61/905,554 filed Nov. 18, 2013,the contents of which are incorporated herein by reference.

STATEMENT OF GOVERNMENT INTEREST

The invention was made with United States Government support underContract No. FA8650-11-C-7160 awarded by the Defense Advanced ResearchProjects Agency (DARPA). The United States Government has certain rightsin this invention.

BACKGROUND 1. Field

The present system relates to spectrum sensing and more particularly tospectrum sensing by means of tunnelized cyclostationary processing. Thecurrent invention relates generally to apparatus, systems and methodsfor spectrum sensing. More particularly, the apparatus, systems andmethods relate to wireless systems signal detection, signalcharacterization and more efficient use of spectrum. Specifically, theapparatus, systems and methods provide for low-energy implementation ofcyclostationary spectrum sensing algorithms disclosed herein as“Tunnelized Cyclostationary Processing”.

2. Description of Related Art

Spectrum sensing has been widely used for cognitive radio systems.Techniques that exploit second and higher order cyclostationary featuresto detect and classify signals are highly desirable. However, theirpervasive use and hardware implementation consumes a lot of energy. Asignificant amount of research has been conducted in the area ofspectrum sensing. However, the underlying assumption of a majority ofthese techniques has been the use of energy detection which performspoorly in dynamic interference environments and cannot perform signalclassification. Second and higher order cyclostationary based sensinghelps in distinguishing between signals and detecting anomalies.However, a majority of these approaches are prohibitively expensive interms of computation.

SUMMARY

In one aspect, the system is a spectrum sensor with an RF front end andprocessing unit that comprises under sampling an input signal to takesamples in one or more tunnels; detecting one or more cyclostationaryfeatures of the input signal based on the samples; and determining oneor more signal types of the one or more cyclostationary features basedon the one or more cyclostationary features.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

One or more embodiments are set forth in the drawings and in thefollowing description. The appended claims particularly and distinctlypoint out and set forth the invention.

FIG. 1 is a diagrammatic view of a data collection and processing deviceor spectrum sensor which may be used to effect the various methodsdescribed herein.

FIG. 2 is a graph which illustrates the concept of tunneling.

FIG. 3A is a flow diagram showing signal processing flow for known andunknown signal detection and characterization.

FIG. 3B shows another instantiation of the signal processing flow.

FIG. 3C is a flow diagram showing Stage 3 spectral correlation functionestimation for known type signals types using their known or previouslyestimated cycle frequencies.

FIG. 4A illustrates Non-conjugate and the Conjugate Spectral CorrelationFunctions (SCF) for the entire analysis bandwidth.

FIG. 4B is a graph showing non-conjugate spectral correlation functionprocessing for known signal types, where the system only tries to detectcertain known Cycle Frequencies (CF).

FIG. 4C further elaborates the difference between Basic Tunneling andDual Tunneling where the Basic Tunneling is used to detect CFs less thanor equal to the Tunnel Bandwidth (BW) and Dual Tunneling is used todetect CFs greater than or equal to the Tunnel BW.

FIGS. 5A, 5B, 5C and 5D show graphs illustrating Stage 3 known typesignal detection and characterization using “tunneling”.

FIG. 6A shows the concept behind unknown signal detection using blind CFestimation to find the Detected Frequency Interval (DFI).

FIG. 6B shows Stage 4 spectral correlation function estimation forunknown signal types using strip spectral correlation analyzer (SSCA).

FIGS. 7A, 7B and 7C show graphs illustrating Stage 4 and Stage 5spectral correlation function estimation for unknown signal types usingSSCA.

FIG. 8A shows Stage 5 processing using second and higher ordercyclostationary processing where Stage 5 has been further broken downinto Levels 0 to 3.

FIG. 8B shows the process of further analyzing the DFI by analyzing theprincipal domain by dividing it into subdiamond regions.

FIG. 8C illustrates that not all the diamond-like regions need to beanalyzed for blind classification of the unknown-types of signal.

FIG. 9A shows Stage 5 Level 0 and Level 1 Cycle Frequency Patternanalysis to recognize various modulation formats. The Signal to NoiseRatio (SNR) is 3 dB in this illustration.

FIGS. 9B and 9C show Stage 5 higher order cyclic cumulants (HOCC) formodulation recognition where second order signal processing may not besufficient.

Similar numbers refer to similar parts throughout the drawings.

DETAILED DESCRIPTION

A data collection and processing device or spectrum sensor is showngenerally at 1 in FIG. 1 and is in a broad sense configured to sample,detect and classify radio frequency signals. Device 1 may include aradio frequency (RF) front end/receiver 2 in communication with one ormore antennae 4, a signal processing unit 6 in communication with frontend/receiver 2, and a display 8 in communication with signal processingunit 6. Device/sensor 1 may also include or be in communication withanother system 10, which may be in communication with the signalprocessing unit 6. It will be understood by one skilled in the art thatdevice/sensor 1 may be configured in any suitable form capable ofeffecting the various methods described herein.

RF front end/receiver 2 essentially provides a receiver function and iscapable of tuning to a certain frequency band and collecting the signalover a certain desired bandwidth. Front end/receiver 2 may include ananalog to digital (A/D) converter for converting the analog signal to adigital signal. Front end/receiver 2 may also include an automatic gaincontrol circuit for automatically controlling the gain of a givensignal.

Signal processing unit 6 is configured for signal processing thatcontains the spectrum sensing algorithms discussed in greater detailfurther below. Processing unit 6 may include a computer program storedon or run by hardware or software for implementing this signalprocessing, such as on a general purpose processor. The hardware may,for instance, be or include one or more of digital signal processing(DSP), a field programmable gate array (FPGA), an application specificintegrated circuit (ASIC), and a graphics processing unit (GPU).

Display 8 is configured to display information received from unit 6. Forinstance, display 8 may display signals that are detected/classified (asdiscussed further below) and communicated to display 8. Display 8 may beany type of display known in the art, for example, a computerdisplay/monitor, a cell phone display, a personal digital assistant(PDA) display and so forth.

Other system 10 may have any variety of configurations which may receivepertinent information from unit 6 for various purposes. Spectrum sensingprovided by signal processing unit 6 to system 10 can provide valueadded information to system 10 for its operation in many cases. Forexample, other system 10 may include a cognitive software defined radio(SDR) system whereby processing unit 6 communicates to the SDR systemwhere unused spectrum is, so that this cognitive radio system may usethat unused spectrum for communications purposes. System 10 may alsoreceive information from unit 6 for such applications as, for example,quantification of the available spectrum through spectrum observatories,on-demand spectrum survey and report, collaborative spectrum measurementand calibration, labeling of systems utilizing the spectrum, spectrumplanning, spectrum mapping, coverage analysis for wireless deployment,terrain and topology—shadowing and fading analysis, complementing thedatabase access for spectrum sharing by adding in-situ awareness andfaster decision making, Space-Time-Frequency spectrum holeidentification and prediction where non-time-sensitive tasks can beperformed at certain times and at certain locations when the spectrumuse is sparse or non-existent, and identification and geo-location ofinterference sources, etc.

The system and method(s) herein provide novel tunnelized second andhigher-order cyclostationary processing algorithms to simultaneouslydetect and characterize signals. As previously noted, techniques thatexploit second and higher order cyclostationary features to detect andclassify signals are highly desirable. However, their pervasive use andhardware implementation have been hampered because such algorithms arehighly complex, and consume a lot of energy. The inventors havediscovered that severe but purposeful under-sampling of the signalsthrough tunneling (explained herein) preserves the cyclostationarity ofthe signals, even when the tunnel bandwidth is much smaller than thesignal bandwidth. This phenomenon is then exploited to create a lowcomplexity and flexible suite of algorithms to simultaneously detect andclassify or characterize the signals using their cyclostationaryfeatures. As demonstrated herein, such algorithms can detect andcharacterize signals under a highly adverse signal to interference plusnoise ratio, even when multiple signals are co-channel andsimultaneously interfering with each other.

In general, any spectrum sensing related processing involves some formof data collection device or spectrum sensor such as device/sensor 1. Aspartially discussed above, this data collection device may include oneor more antennas, radio frequency (RF) receivers that tune to aparticular frequency, followed by amplifiers and mixers to amplify anddown-convert the signal for processing, and finally analog to digital(A/D) converters that sample the analog signal at a nominal samplingfrequency and digitize them for processing. This processing that happensin general for any spectrum sensor is well understood. The focus of ourinvention is the processing that happens on this digitized data tofurther understand what kind of signals are actually operating in theanalysis bandwidth, as well as a system, apparatus or device/sensor 1which is configured to effect this processing/the method(s) describedherein. The analysis bandwidth is defined as the total bandwidth whichneeds to be analyzed by the spectrum sensing algorithms at a time.Typically, the analysis bandwidth is less than or equal to the samplingrate (fs) to meet the Nyquist criteria. Device/sensor 1 may be used toeffect method(s) herein which may include under sampling an input signalto take samples in one or more tunnels each with a tunnel bandwidth thatis equal to or less than a total analysis bandwidth of the input signal;detecting one or more cyclostationary features of the input signal basedon the samples; and determining one or more signal types of the one ormore cyclostationary features based on the one or more cyclostationaryfeatures.

FIG. 2 illustrates an embodiment of the concept of “tunneling”, which ispurposeful under sampling of the analysis bandwidth spectrum intosmaller sub-bands called tunnels. These tunnels could be a result of asimple channelizer circuit, or of something more sophisticated such ascompressive sensing or sampling. In particular, FIG. 2 shows the powerspectral density (PSD) for four wideband signals with no spectraloverlap. The four wideband signals shown as examples are respectivelyGlobal System for Mobile (GSM), Direct sequence spread spectrum (DSSS),frequency hopping and direct sequence spread spectrum (FH-DSSS) or(FH/DS SS), and orthogonal frequency division multiplexing (OFDM).

The invention provides computationally efficient cyclostationarydetection and classification algorithms which do not require processingof full signal bandwidth. As shown in FIG. 2, we formulated this novelapproach called “tunneling” where we discovered that severelyunder-sampled signals (although distorted) retain exploitablecyclostationary properties where these tunnelized signals can bedetected and characterized with the same performance as traditionalcyclostationary algorithms. The exploitable cycle frequencies (CFs) inspite of tunneling are manifested as a result of repeating features ofthe signal that originate from the structures associated with framesize, packet size, preamble structure, pilot patterns and hop patterns,as well as fast moving features that are associated with symbol rate orchip rate. FIGS. 3A and 3B describe the signal processing flow for thetunnelized cyclostationary processing to detect known and unknown signaltypes.

The ability of the tunnelized cyclostationary algorithms to provideenergy savings of greater than ten times that of conventionalcyclostationary algorithms comes from the fact that instead of analyzinga high sample rate input signal stream, one can now process a severelysub-sampled tunnelized signal stream and apply striding, where not allthe tunnels are analyzed, but instead many of them are skipped at nopenalty in performance.

FIGS. 3A and 3B show the signal processing flow of the Cognitive radioLow-energy signal Analysis Sensor Integrated Circuit (CLASIC)algorithms. The CLASIC algorithms are implemented in five stages suchthat the sophistication and complexity of the algorithms increase witheach stage. The various stages are labeled in FIG. 3B and shownrespectively at blocks 12, 14, 16, 18 and 20 in FIG. 3A. In Stage 1(block 12), channelization or tunneling is conducted, and an exhaustivespectrum survey is also conducted to detect (blind detection) all typesof signals (narrowband and wideband) that are well above the noisefloor. In Stage 2 (block 14), narrow-band signals are detected andcharacterized using second and higher order cyclostationary processing.In Stage 3 (block 16), known types of signals are detected andcharacterized using their second order cyclostationary properties. Thismay involve semi-blind low-SNR wideband signal detection, separation andclassification based on second and higher order cyclostationaryanalysis. Stage 4 (block 18) of the process is used to detect unknowntypes of signals and find the detected frequency interval (DFI) which isan approximate frequency support of the signal. This may involve blindwideband signal detection and second order cyclostationary analysis.Stage 4 results are passed on to Stage 5 (block 20), which serves todetect and characterize the unknown signal type using second and higherorder cyclic cumulants (HOCCs). This may involve blind wideband signalseparation and classification based on second and higher ordercyclostationary analysis.

Stage 1—Tunneling the Input Sample Stream

The function of Stage 1 is to perform tunneling of the input datastream. In Stage 1, device/sensor 1 may be used to under sample an inputsignal to take samples in one or more tunnels each with a tunnelbandwidth that is equal to or less than a total analysis bandwidth ofthe input signal. A channelizer may be used with a sufficiently longwindow to create enough isolation between tunnels to be able towithstand interference from other higher power signals, which may besignals with highly dynamic characteristics. For example, thechannelizer may be a polyphase windowed discrete Fourier transform (DFT)channelizer. The polyphase windowed DFT channelizer may be followed byan energy detection stage performed by applying a block averagingcircuit to each tunnel over time to obtain accurate estimates of thenoise floor of the given tunnel and hence detect signals that aresufficiently above the noise floor to perform rapid analysis of the RFscene. This is equivalent to performing Energy Detection (ED) for thespectrum, with frequency resolution equal to the bandwidth of the giventunnel. Finally, tunnelized signals are provided to a whitener circuit,which performs the function of creating a level playing field for allsignals and normalizes it to an average amplitude of unity, no matterwhat their input received power may be, which allows signals with a widevariety of cyclostationary properties to be detected without causingfalse alarms or missed detection for other signals. The other advantageof the whitener is to scale the signal without losing any informationwhich ensures that the maximum or the minimum bit-widths for digitalsignal processing (DSP) in hardware implementation do not go out ofproportion, even when the signal suffers from excessive low or highswings. If this step is skipped, then it is difficult to set aparticular threshold when a wide variety of signals need to be analyzed.

Stage 3—Simultaneous Detection and Characterization of Known Type ofSignals

The function of Stage 3 is to perform detection and characterization ofknown types of signals using their known or previously estimated cyclefrequencies (CFs). The primary algorithm that is used to determine thecyclostationary property of the signals is given by the well-knownEquations (1) below that represent the cyclic periodogram for the p^(th)tunnel and processing of N samples for a given CF α, and for thefrequency support f. Xp and Xq correspond to the discrete Fouriertransforms (DFT) of the whitened signals for the pth tunnel and the qthtunnel respectively.

X _(p,N)(k,f)=FFTN(x _(p)(t−kN))

FFTN( ) denotes N point Discrete Fourier Transform (DFT) processing.X_(p,N) and X_(q,N) correspond to the p^(th) and q^(th) tunnel signalsafter DFT processing. Parameter α is cycle frequency, parameter f issupport frequency and parameter M is the number of blocks with eachblock containing N point DFT output. I_(x) _(p) _(x) _(q) ^(α) is thegeneral spectral correction function with cycle frequency α and isobtained by p^(th) and q^(th) tunnel signals. S_(x) _(p) _(x) _(q) ^(α)is the spectral correction function after time smoothing by averaging Mcoherent cyclic periodograms.

$\begin{matrix}{{{I_{x_{p}x_{q}}^{\alpha}( {k,f} )} = {{X_{p,N}( {k,{f + \frac{\alpha}{2}}} )}{X_{q,N}^{*}( {k,{f - \frac{\alpha}{2}}} )}}}{{{\hat{S}}_{x_{p}x_{q}}^{\alpha}(f)} = {\frac{1}{NM}{\sum\limits_{k = 0}^{M - 1}{{I_{x_{p}x_{q}}^{\alpha}( {k,f} )}e^{{- i}\; 2{\pi\alpha}\; {kN}}}}}}} & {{Equations}\mspace{14mu} 1}\end{matrix}$

The cyclic periodogram, when averaged over many data records, with avalid phase factor, produces an estimate of the spectral correlationfunction (SCF) which has the (f,α) support (also known as the principaldomain) that resembles a diamond.

FIG. 4A shows an illustration of the SCF of an entire or total analysisbandwidth. With reference to FIG. 4A, it is noted that all signals andnoise contribute to the power spectrum (α=0), but different signalscontribute to different parts of the spectral correlation planes. Whatis difficult to detect and sort in the power spectral density (PSD) canbe easy in the planes. Signal parameters reflected in spectralcorrelation can be estimated in the presence of interference.

FIG. 4B illustrates how we perform our Stage 3 processing to only detectcertain known CFs for the signals that we wish to detect. As shown inFIGS. 4B and 4C, if the CF of a signal is known a priori, then it is notnecessary to compute the SCF exhaustively. Rather, one can significantlyreduce the complexity by simply looking for the SCF response for theknown CFs and comparing the response against a threshold for noise andother signal types. This results in significant reduction incomputational cycles involved. Further, as mentioned below, not alltunnels need to be analyzed for signals. Depending upon the size of thetunnels and the signals that need to be detected, we apply a stridingfactor, which allows a significant number of tunnels to be skippedwithout any performance penalty.

Three types of cyclostationary statistics are computed in Stage 3.“Basic” tunnel response corresponds to the processing where the CFs tobe found are smaller than the tunnel bandwidth. Hence, both the Xp andthe Xq terms in Equation (1) come from the complex baseband samples ofthe same tunnel, or in other words, p=q. As an example, a Wideband CodeDivision Multiple Access (WCDMA) signal contains strong CFs at 15 kHz,30 kHz and 45 kHz, which in our case belongs to Basic tunnel processing.“Dual” tunnel response corresponds to the processing where the CFs to befound are greater than the tunnel bandwidth. Hence, both the Xp and theXq terms in Equations (1) come from the complex baseband samples of twodifferent tunnels, where the distance between the two tunnels is roughlyequal to the CF that needs to be detected. As an example, a WCDMA signalthat contains strong CFs at 3.8488 MHz would belong to Dual tunnelprocessing.

Both the Basic and Dual tunnel responses correspond to a non-conjugatecyclic periodogram, where a conjugate operation is applied for the Xqterm in Equation (1). However, when this conjugate term is missed, andthe CF to be detected is greater than the tunnel bandwidth, then itresults in the statistic that is referred to as the “Dual Conjugate” asshown in Equation 2:

$\begin{matrix}{{{\hat{I}}_{x_{p}x_{q}^{*}}^{\alpha}( {t,f} )} = {\frac{1}{N}{X_{p,N}( {t,{f + \frac{\alpha}{2}}} )}{X_{q,N}( {t,{\frac{\alpha}{2} - f}} )}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

The dual conjugate is used to detect signals that have no knownnon-conjugate CFs (e.g., Advanced Television Standards Committee DigitalTelevision or ATSC DTV signal), but contain CFs that are proportional totheir carrier frequency and the symbol rate. As can be seen fromEquation 2, not applying the conjugation factor retains the doublecarrier terms in the SCF.

FIGS. 5A, 5B, 5C and 5D show one of the extraordinary results of thesystem and process presented herein. This is the case where five signals{viz. Global System for Mobile (GSM), Wideband Code Division MultipleAccess (WCDMA), unknown type, Advanced Television StandardsCommittee-Digital Television (ATSC-DTV), 3rd Generation PartnershipProject (3GPP) Long Term Evolution (LTE) (5 MHz)} are co-channel andinterfering with each other, as shown in FIG. 5A. Using Basic tunnelprocessing (FIG. 5B), Dual tunnel processing (FIG. 5C) and DualConjugate tunnel processing (FIG. 5D), it is possible to detect andclassify all the known type signals in spite of them being right on topof each other and in extremely poor signal to interference plus noiseratio (SINR) environment. The FIG. 5A graph illustrates basic tunnelbased detection and classification, and more particularly showsdetection and classification of GSM, WCDMA and LTE5M using basictunneling; the FIG. 5B graph illustrates dual tunnel based detection andclassification, and more particularly shows detection and classificationof WCDMA using dual tunneling; and the FIG. 5C graph illustrates dualconjugate tunnel based detection and classification, and moreparticularly shows detection and classification of ATSC-DTV using dualconjugate tunneling.

Table I illustrates the confusion matrix for the scene represented inFIGS. 5A-5D for 100 trials, where each signal had an in-band signal tonoise ratio (SNR) of 3 dB. As can be seen from Table I, the proposedtunnelized cyclostationary algorithms are able to detect andcharacterize the signals with nearly ideal performance.

TABLE 1 STAGE 3 DETECTION AND CHARACTERIZATION - CONFUSION ClassifiedAs: In-band LTE SNR = 3 dB WCDMA GSM (5 MHz) DTV Input WCDMA 100% SignalGSM 100% LTE 100% (5 MHz) DTV 100%

Some other things of note are that, during experimentation, theinventors were able to create significant reduction in computationalfootprint by using partial fast Fourier transform (FFT) points forSignals Xp and Xq in Equations (1) and (2). As an example, the FFT sizemay be 16, the analysis bandwidth may be 50 MHz which when divided into256 tunnels results in each tunnel having a bandwidth of 195.3125 kHzeach. The spectrum sensing time for the results shown in FIGS. 5A-D andTable I was 160 milliseconds (ms), or approximately 2²³ samples if thesampling rate (fs) is 50 MHz.

IV. Stage 4—Unknown Signal Detection

Stage 4 includes unknown type of signal detection and detected frequencyinterval (DFI) computation. This is accomplished using bothcyclostationary processing and energy detection. Since it is notpossible to know the characteristics of the unknown signal, anexhaustive CF search for the tunnels is performed. The criteria used isto detect at least one CF in a given tunnel. FIG. 6A illustrates theconcept of Stage 4 unknown signal type detection where tunnels withsimilar CFs are grouped together to create a DFI, which is then providedto the Stage 5 processing for further analysis.

Computing the spectral correlation function (SCF) using Equations (1)and (2) is efficient when the CFs are known a priori, but not veryefficient when one does not know the CFs, as disclosed by W. A. Brownand H. H. Loomis, Jr., in Digital Implementations of SpectralCorrelation Analyzers, IEEE Transactions on Signal Processing, Vol. 41,No. 2, February 1993, pp. 703-720.

The below-noted algorithm, which is extremely hardware friendly, dividesthe 2-D (f,α) plane into diagonally arranged strips as shown in FIG. 6B,which for instance shows a strip 6 and a strip 8 (of eight strips intowhich the plane might, for example, be divided). Once again, it is notnecessary to analyze each and every strip in each and every tunnel. Inthe majority of cases, a few strips are enough to detect the signal withreasonable probability of detection (Pd) and probability of false alarm(Pfa).

The generic strip spectral correlation analyzer (SSCA) algorithm may bedescribed using the following Equations (3):

$\begin{matrix}{{{{\hat{S}}_{x_{N}}^{\alpha}(f)} = {\sum\limits_{r = 0}^{N - 1}{{X_{N\; \prime}( {r,{f + \frac{\alpha}{2}}} )}{x^{*}(r)}e^{{- i}\; 2\pi \; {r{({\frac{\alpha}{2} - f})}}}}}}{{X_{N\; \prime}( {r,f} )} = {\sum\limits_{j = {{- N}/2}}^{\frac{N\; \prime}{2} - 1}{{\alpha (j)}{x( {r - j} )}e^{{- i}\; 2\pi \; {f{({r - j})}}}}}}} & {{Equations}\mspace{14mu} (3)}\end{matrix}$

where two fast Fourier transforms (FFTs) of respective sizes N′ and N(i.e., a first size FFT N′ and a second size FFT N) are used or defined,wherein N is the data length or spectrum sensing length, and N′ is asmaller than N and may be much smaller than N. Ŝ_(x) _(p) _(x) _(q) ^(α)is a strip spectral correction function obtained by N samples. α iscycle frequency and f is support frequency. x(r) is a time domain signalbeing processed and a(•) are the coefficients of data-tapering windowrespectively.FFT N′ may also be called a channelizer. All this results in acomprehensive computation of the spectral correlation function over the(f,α) principal domain. FIG. 6A shows the implementation of the SSCAusing N′ point and N point FFTs. Two input sample streams are providedto the SSCA. If the sample streams originate from the same tunnel, i.e.,when k=p, the resultant SSCA is termed as Auto SSCA. When kth and pthwhitener outputs to the SSCA come from two different tunnels, then theresulting SSCA is termed as the Cross SSCA. Stage 4 may use Auto orCross SSCA. Stage 5 uses both the Auto and the Cross SSCA computation.For example, N′ may be 8, N may be 2¹⁵ or approximately 32,000 samples,which corresponds to approximately 160 ms of sensing at 50 MHz ofsampling and 256 tunnels.

Referring to FIG. 6A, a regularly spaced set of tunnels may be used withS>1. For each visited tunnel, the full non-conjugate SCF may be computedusing the SSCA algorithm. Significant CFs may be detected using thespectral coherence statistic. Spectral bands containing non-noise energymay be identified by grouping together adjacent (modulo striding)tunnels containing CFs. The signal at 28 in FIG. 6A will be detected,whereas the signal at 30 in FIG. 6A will be missed. FIG. 6A shows at 32intervals which may be blindly estimated. To blindly estimate cyclefrequencies that indicate the presence of non-noise RF energy, spectralcorrelation analysis is applied to a set of sparsely sampled tunnels,resulting in the subdiamonds near the frequency axis in the left plot ofFIG. 6A. The SSCA is used to exhaustively analyze all the cyclefrequencies that can appear in each subdiamond shown in the left plot ofFIG. 6A.

FIGS. 7A, 7B and 7C illustrate the performance of the Stage 4 processingusing Basic or Auto SSCA processing for a single strip. The graph inFIG. 7A illustrates the input scene with an unknown signal type. Thegraph in FIG. 7B illustrates the number of new CFs detected for thetunnels occupied by the unknown signal type, and more particularly showsat 22 detected frequency intervals (DFI) with similar CF content, and at24 that ATSC video carrier and GSM show very strong CF content. Thegraph in FIG. 7C illustrates the DFI region, which is the region that isfurther subjected to Stage 5 processing for further analysis. The graphin FIG. 7C shows at 26 the unknown signal and its region of supportdetected using Stage 4 processing.

Stage 5—Unknown Signal Characterization Using Second and Higher OrderCyclic Cumulants

Stage 5 is the most complicated stage of our CLASIC circuit since itneeds to process the DFI regions for unknown signal types provided byStage 4, and not only characterize the signal type, but also identifyits modulation. Hence, the Stage 5 processing is broken down into fourlevels (0, 1, 2 and 3) as shown in FIG. 8A. Although Stages 1 through 4as well as Stage 5 Level 0 and 1 are processed after tunneling, it isnot necessary to perform tunneling for Stage 5 Level 2 and 3. Stage 4provides the estimate of the carrier frequency and the DFI of thedetected signals to Stage 5, where the Second and Higher Order analysisis then performed on a portion of the band that is known to contain thesignal.

FIG. 8A illustrates the details of Stage 5 Levels 0 through 3 and howthese Levels are used to provide signal characteristics understandingfrom the lowest fidelity to the highest fidelity. FIG. 8B shows theprocess of further analyzing the DFI by analyzing the principal domainby dividing it into smaller diamonds or subdiamonds or subdiamondregions (here, 49 subdiamonds). FIG. 8B illustrates that one way toobtain the CF pattern is exhaustive estimation of SCF using only tunnelsthat span the DFI. If all subdiamonds are analyzed by performing crossspectral correlation analysis for all possible pairs of tunnels, thesubdiamonds will tile the DFI's principal domain, as shown in the leftplot or graph of FIG. 8B. A similar analysis can be performed for theconjugate spectral correlation function, again using all possible tunnelpairs, as illustrated in the right plot or graph of FIG. 8B.

FIG. 8C illustrates that not all the subdiamond regions need to beanalyzed for blind classification of the unknown types of signal. Thus,a selected subset of tunnels may be analyzed to infer the CF pattern.For example, the non-conjugate spectral correlation graph on the left ofFIG. 8C shows that 19 of the 49 subdiamonds of FIG. 8B may be analyzedfor this purpose, and the conjugate spectral correlation graph on theright of FIG. 8C shows that 15 of the 49 subdiamonds of FIG. 8B maylikewise be analyzed. Use of only some tunnel pairs in the crossspectral correlation analysis can still provide sufficient informationto detect and classify most RF signals. FIG. 8C shows a particularsampling of all possible tunnel pairs for both the non-conjugate andconjugate spectral correlation function. This particular sampling placessubdiamonds in regions near the frequency and cycle frequency axes,which are the regions mostly likely to contain important cyclefrequencies.

Stage 5 Levels 0 and 1 correspond to characterizing the unknown type ofsignal utilizing exhaustive second order SCF analysis using SSCA. UnlikeStage 4, this analysis is much more exhaustive and involves not onlycomputing all the CFs, but also their patterns. The SCF for any CF inthe original high-rate data can be estimated from tunnels by usingappropriate auto or cross SCF estimators. Auto non-conjugate SCFestimates cover the CFs near the frequency axis. Auto conjugate SCFestimates cover the CFs near the CF axis. Cross SCF estimates fill inthe remainder of the two diamonds. This has been illustrated using FIG.4C.

The large diamond on the left of FIG. 4C illustrates the wideband datascenario; this is the aerial view of the spectral correlation functionfor the wideband data prior to tunneling. The lines inside the diamondindicate cycle frequencies for various signals. The cycle frequenciesthat are known in advance of processing are shown by dashed lines. Thelarge diamond on the right of FIG. 4C illustrates basic and dualtunneling. Each subdiamond on the plot represents the spectralcorrelation function that can be estimated by performing a crossspectral correlation analysis between two tunnels. If all pairs areconsidered, the large diamond is tiled by the smaller diamonds. Theindices inside the subdiamonds indicate the involved tunnels, assumingthat there are eight tunnels total. A single number indicates that thecross operation defaults to an auto operation. The Basic Tunnelingemploys only tunnel pairs that result in the subdiamonds near thefrequency axis; this applies to all known cycle frequencies that havemagnitude less than the tunnel bandwidth. On the other hand, DualTunneling employs other pairs of tunnels, resulting in the subdiamondsshown well above the frequency axis.

Stage 5 Levels 2 and 3 conduct further analysis to separate themodulations of various signal types. For example, as shown in FIG. 8A,16-QAM (quadrature amplitude modulation) and π/4 DQPSK (differentialquadrature phaseshift keying) have identical second ordercyclostationary features, so they cannot be distinguished based onsecond order statistics alone. However, their higher order features,computed from the higher order cyclic cumulants (HOCC) show remarkabledifference in their patterns. Hence, the HOCCs can be effectively usedto distinguish between the modulations of various unknown types ofsignals.

What makes the Stage 5 implementation complex is the fact that HOCCs ofup to the sixth order may need to be computed. The order implies thenumber of terms to be multiplied. For example, a Sixth Order term isobtained by multiplying the signal six times. The HOCCs are thensynthesized using various combinations of these n^(th) order terms wherethe constituent signals may or may not be conjugated. The basic equationfor the higher order cyclic temporal moment function (CTMF) is given byEquation (4).

$\begin{matrix}{{{\hat{R}}_{x_{d}}^{\alpha}( {\tau,n,m} )} = {\frac{1}{2K}{\sum\limits_{k = {- K}}^{K - 1}{\lbrack {\prod\limits_{j = 1}^{n}{x_{d}^{{(*})_{j}}( {k + \tau_{j}} )}} \rbrack e^{{- i}\; 2{\pi\alpha}\; k}}}}} & {{Equation}\mspace{14mu} (4)}\end{matrix}$

Here, α is the CF of the higher order cyclic moment function, x_(d) isthe detected unknown signal, τ and τ_(j) are the lag factors, n is theorder and m terms are conjugated. α is cycle frequency and x_(d) is atime domain signal being processed. The symbol (*) denotes whether thecomplex term is conjugated or not, and τ is time offset. The HOCCs arederived from the CTMFs. The derivation becomes significantly complicateddue to tunneling, where the signal x_(d) is then decomposed into itstunneled components.

FIG. 9A shows Stage 5 Level 0 and Level 1 Cycle Frequency Patternanalysis to recognized various modulation formats. The Signal to NoiseRatio (SNR) is 3 dB in this illustration. FIGS. 9B and 9C show Stage 5higher order cyclic cumulants (HOCC) for modulation recognition wheresecond order signal processing may not be sufficient.

FIG. 9A shows that floating point and fixed point implementationscreated by the inventors match well at in-band SNR of −1 dB and we areable to classify the signal accurately as a binary phase-shift keying(BPSK) signal. Once again, ways to significantly reduce thecomputational complexity have been found by only processing a subset ofstrips for each tunnel. One other important aspect that was nothighlighted earlier is the fact that Stage 5 processing involves notonly the auto-SSCA, where the elements for second order SCF come fromthe same tunnel, but also the cross-SSCA, where the element of the SCFcome from different tunnels to find CFs that are greater than the tunnelbandwidth.

FIGS. 9B and 9C show Stage 5 higher order cyclic cumulants (HOCC) formodulation recognition where second order signal processing may not besufficient. HOCCs, in general require extensive computations and areused when second order cyclostationary statistics are not sufficient todiscriminate between signals. An example of this is shown in FIG. 9B,where the second order cyclostationary statistics for 16-QuadratureAmplitude Modulation (QAM) and p/4 Differential Quadrature Phase ShiftKeying (DQPSK) signals have the same patterns. As a result of this, itbecomes nearly impossible to discriminate between these signals based onjust the second order cyclostatioary statistics. Similarly, varioustypes of M-QAM (e. g. 4-QAM, 16-QAM, 32-QAM, 64-QAM) cannot bedistinguished using the second order cyclostationary statistics alone.

FIG. 9C shows the HOCC patterns for 16-QAM and p/4-DQPSK signals. TheY-axis in the figure represents the harmonic number. The Zero line onthe Y axis is the direct current (d.c.) component. +/−1 represents theHOCCs corresponding to harmonics of the signal carrier frequency+/−fc.+/−2 represents the HOCCs corresponding to the harmonics of two timesthe signal carrier frequency (+/−2fc) etc. The x-axis corresponds to theorders of the HOCCs. The variable ‘n’ refers to the order of thestatistic. The term ‘m’ refers to the number of terms that have beenconjugated. For example, (2,0) refers to the second order statistic withno terms conjugated and (2,1) refers to second order statistic with oneterm conjugated. The patterns correspond to the statistics that showmagnitude above a certain set threshold. Based on the HOCC patterns, onecan see that it is possible to distinguish between 16-QAM and p/4-DQPSKmodulation types. According to one embodiment of the present invention,a tunnelized version of the HOCCs was used to determine variousmodulations. In a degenerate case, the tunnel size may be equal to thewidth of the signal bandwidth.

Those skilled in the art will appreciate that we have provided noveltunnelized second and higher-order cyclostationary processing algorithmsto simultaneously detect and characterize signals. Our algorithms showthat severe but purposeful under-sampling of the signals throughtunneling preserve the cyclostationarity of the signals, even when thetunnel bandwidth is much smaller than the signal bandwidth. Thisphenomenon is exploited to create low complexity and flexible suite ofalgorithms to simultaneously detect and characterize the signals usingtheir cyclostationary features. It has also been demonstrated that suchalgorithms can detect and characterize signals under a highly adversesignal to interference plus noise ratio, even when multiple signals areco-channel and simultaneously interfering with each other.

In the foregoing description, certain terms have been used for brevity,clearness, and understanding. No unnecessary limitations are to beimplied therefrom beyond the requirement of the prior art because suchterms are used for descriptive purposes and are intended to be broadlyconstrued. Therefore, the invention is not limited to the specificdetails, the representative embodiments, and illustrative examples shownand described. Thus, this application is intended to embracealterations, modifications, and variations that fall within the scope ofthe appended claims.

What is claimed is:
 1. A spectrum sensor, comprising: a processing unitfor processing digital signals representative of radio frequency signalsprocessed by a radio frequency front end and an analog to digitalconverter, wherein the processing unit comprises at least one processorand a memory section having instructions stored thereon that whenexecuted directs the at least one processor to execute tunnelized secondand higher-order cyclostationary instructions that process the digitalsignals over an analysis bandwidth to determine one or more signal typesbased on one or more second and higher order cyclostationary features,wherein a tunnel bandwidth is less than or equal to the analysisbandwidth.
 2. The spectrum sensor according to claim 1, wherein theradio frequency front end comprises, at least one amplifier, at leastone mixer, at least one filter, and at least one antenna to receive theradio frequency signals and process into down converted signals.
 3. Thespectrum sensor according to claim 2, wherein the analog to digitalconverter processes the downconverted signals into the digital signals.4. The spectrum sensor according to claim 1, wherein the tunnelizedsecond and higher-order cyclostationary instructions perform thefollowing: under sample the digital signal to generate one or moretunnels; identify tunnelized signal samples from the one or moretunnels; whiten the tunnelized signal samples to generate tunnelized andwhitened signal samples; produce a spectral correlation function of thetunnelized and whitened signal samples; and detect the second and higherorder cyclostationary features of the radio frequency signal byanalyzing the spectral correlation function.
 5. The spectrum sensoraccording to claim 1, wherein the processing unit is communicativelycoupled to another system and provides the one or more signal types tothe other system, wherein the other system processes the one or moresignal types to provide at least one of: quantification of availablespectrum, on-demand spectrum survey, spectrum measurement andcalibration, labeling of spectrum systems, spectrum planning, spectrummapping, coverage analysis for wireless deployment, terrain and topologyshadowing, fading analysis, space-time-frequency spectrum holeidentification, and identification and geo-location of interferencesources.
 6. The spectrum sensor according to claim 1, wherein the one ormore signal types are sent to a display, wherein the display is acomputer display, a cell phone display, or a personal digital assistant(PDA) display.
 7. The spectrum sensor according to claim 1, wherein thesignal types are selected from Direct Television (DTV), Global Systemfor Mobile (GSM), Wideband Code Division Multiple Access (WCDMA), andLong Term Evolution (LTE).
 8. The spectrum sensor according to claim 1,wherein the processing unit further comprises series or parallelprocessing of a signal detection unit, a narrow band blind signalclassification unit, a non-blind signal detection unit, a blindfrequency interval detection unit and a blind signal classificationunit.
 9. The spectrum sensor according to claim 8, wherein the signaldetection unit further comprises a channelizer configured to undersample the digital signals.
 10. The spectrum sensor according to claim1, wherein the processing unit is coupled to a Strip SpectralCorrelation Analyzer (SSCA) that is applied to one of more tunnels overthe analysis bandwidth.
 11. A spectrum sensing device, comprising: atleast one antenna; a radio frequency front end coupled to the antennaand receiving radio frequency signals, wherein the radio frequency frontend tunes to a frequency band and produces a downconverted signal; ananalog to digital section for converting the downconverted signal todigitized data; a processing unit for processing the digitized data, theprocessing unit comprising instructions that when executed by theprocessing unit execute tunnelized second and higher-ordercyclostationary instructions to determine one or more signal types basedon one or more second and higher order cyclostationary features; and adisplay for displaying the one or more signal types.
 12. The spectrumsensing device according to claim 11, wherein the radio frequency frontend collects signals over an analysis bandwidth, the tunnelized secondand higher-order cyclostationary instructions comprises under samplingin one or more tunnels having a respective tunnel bandwidth, and whereinthe tunnel bandwidth is less than or equal to the analysis bandwidth.13. The spectrum sensing device according to claim 11, wherein thedisplay is a computer display, a cell phone display, or a personaldigital assistant (PDA) display.
 14. The spectrum sensing deviceaccording to claim 11, wherein the processing unit is further configuredto: under sample an input signal to generate one or more tunnels;identify tunnelized signal samples from the one or more tunnels; whitenthe tunnelized signal samples to generate tunnelized and whitened signalsamples; produce a spectral correlation function of the tunnelized andwhitened signal samples; and detect the second and higher ordercyclostationary features by analyzing the spectral correlation function.15. A non-transitory computer readable medium storing instructionsexecutable by a processing device, the instructions implementing amethod to classify radio frequency signals, execution of theinstructions causes the processing device to: under sample an inputsignal to generate one or more tunnels; identify tunnelized signalsamples from the one or more tunnels, wherein the tunnelized signalsamples have a bandwidth that is equal to or less than an analysesbandwidth of the radio frequency signals; whiten the tunnelized signalsamples resulting in tunnelized and whitened signal samples; produce aspectral correlation function of the tunnelized and whitened signalsamples; and detect a set of second and higher order cyclostationaryfeatures of the radio frequency signals by analyzing the spectralcorrelation function.
 16. The non-transitory computer readable mediumaccording to claim 15, wherein the execution of the instructions furthercauses the processing device to determine one or more signal types fromthe set of second and higher order cyclostationary features.
 17. Thenon-transitory computer readable medium according to claim 16, whereinthe processing device is communicatively coupled to another system andprovides the one or more signal types or the set of second and higherorder cyclostationary features to the other system, wherein the othersystem processes the one or more signal types to provide at least oneof: quantification of available spectrum, on-demand spectrum survey,spectrum measurement and calibration, spectrum labeling of systems,spectrum planning, spectrum mapping, coverage analysis for wirelessdeployment, terrain and topology shadowing, fading analysis,space-time-frequency spectrum hole identification, and identificationand geo-location of interference sources.
 18. The non-transitorycomputer readable medium according to claim 15, wherein the one or moresignal types are displayed on a display.
 19. The non-transitory computerreadable medium according to claim 15, further comprising: transmittinga signal based on the one or more signal types.